How Late is Too Late for Corn?

By: Harold Watters, Ohio State University Extension CCA

As I write this it is obvious that the majority of the corn crop this year will be planted after May 20. I sat last Thursday with a grower from Miami County. We figured the days it takes him to dry out, then to plant first corn and then soybeans and determined that at least some of his crop will be planted into June no matter what. Yields are likely to be reduced. We do know that with good growing conditions and timely late-season rains, we can still produce a decent crop. Consider the economics of your decisions during this season, make those applications that can make you money and skip those that only make you feel good.

Frost worries? Or just wet corn? The corn plant has the ability to adapt to later planting by advancing more rapidly through the growth stages. Work done at Purdue and Ohio State by graduate students of Bob Nielsen and Peter Thomison, show that the number of growing degree days (GDD) needed from planting to maturity decreases by about 7 GDD per day of delayed planting. So a hybrid planted on May 30 needs about 200 less GDDs to achieve maturity than a hybrid planted on May 1. I think this ability of the corn plant to adapt is why we can still have decent yield expectations.

We don’t have much nitrogen on yet for corn, and some are even struggling to get it on their wheat. In corn, our recommendation is to evaluate the crop and look for any visual symptoms of nitrogen deficiency (general chlorosis or yellowing). If you somehow applied early N, but not sure how much is left you can use the worksheet from our Corn, Soybean, Wheat and Forages Field Guide (page 81, and here below) to evaluate the risk of nitrogen loss.

1) What N source was utilized?

1 point – Anhydrous ammonia with nitrification inhibitor

2 points – Anhydrous ammonia

3 points – Other fertilizer banded

4 points – Other fertilizer broadcast


2) When was the N applied?

2 points – After April 20

5 points – Before April 20


3) How much N has been applied?

1 point – >200 lbs/A

2 points – 150-200 lbs/A

3 points – 100-150 lbs/A

6 points – <100 lbs/A


4) What has been the predominant soil moisture status in the field this spring?

1 point – Normal

2 points – Wet

4 points – Excessively wet (saturated – standing water)


5) What is the crop’s condition?

1 point – Green plants > 12” tall

2 points – Green plants < 12” tall

3 points – Chlorotic plants < 12” tall

5 points – Chlorotic plants > 12” tall


Total the score and use the following guidelines:

Less than 13 – Additional fertilizer not recommended

13-16 – Evaluate again in 4-7 days

17 or greater – Add an additional 40-70 lbs N/A

Some producers may consider the use of the pre-sidedress soil nitrate test (PSNT) to determine if additional N fertilizer is warranted. This can work well if manure is in your mix, or you did apply pre-plant anhydrous. To attain a representative soil sample, collect 15, 1-foot deep random cores from a field and mix them thoroughly. Submit a grab sample from the composite to a reputable lab. You may want to contact the lab to learn their turn-around time — you want it back quickly.

  • If the nitrate level in the soil is between 25 to 30 ppm then no additional N is warranted.
  • If nitrate levels are lower than 15 ppm then normal N rates should be applied.
  • If between 15 and 30 ppm then a reduced rate is likely all that is needed. Some suggest 50 to 70 pounds of nitrogen per acre additional would make sense.

Got manure still in the lagoon? It can provide an excellent source of nitrogen. Consider top-dressing or side dressing that nutrient source. See this Ag BMP here:


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